Polaris Ultra RMK 800bb head cc

[Samikoo]

Hi!

I´m currently building a big bore Ultra(-96 RMK) with stock RMK Ultra heads. Stock cylinders sleeved for Storm pistons (Wiseco).

Since the RMK heads have a smaller dome than regular Ultras, where should I be aiming for cc-wise? (this assumption on "tighter" heads is solely based on SLP:s information on pipe tuning for the 680 Ultra)

What kind of cc´s do 800 big bore kit heads have? Or is there any need for head modification at all? I will be running it at max 3000ft.

 

[Throttle Junkie]

Send engine-tech a PM he built thousands of the 800BB and might help you out.

Also, I advised to stay away from Siezcos, you will have nothing but cold seizures with the steel cylinders. I ran Kimpex coated 800 storm pistons in mine for 7 years with great luck.

 

[Scavenger]

Sounds very good! The ultra is a real contender, especially with a good BB kit.
You should aim for a comb.chamber volume of 24cc:s. It will give you a UCCR of ~12:1 which is plenty safe. Put the squish area ratio at 42% and the clearande @ the bore edge at 1mm with a 12* squish angle and you will be where you need to be. Then play with the timing until you find max. power, start with it at 12*btdc@8000 rpm and try moving up towards 14* on the dyno.

 

[saskrider]

ultra heads

Hi!

I´m currently building a big bore Ultra(-96 RMK) with stock RMK Ultra heads. Stock cylinders sleeved for Storm pistons (Wiseco).

Since the RMK heads have a smaller dome than regular Ultras, where should I be aiming for cc-wise? (this assumption on "tighter" heads is solely based on SLP:s information on pipe tuning for the 680 Ultra)

What kind of cc´s do 800 big bore kit heads have? Or is there any need for head modification at all? I will be running it at max 3000ft.

If you need another set of stock ultra heads,I have a set in new condition.
GOOD LUCK with the build.

 

[Samikoo]

Thanks for the answers so far!

So I´ll propably go for .040-.060 squish, since altitudes stay below 3000ft, on premium pump gas. I´ll have to measure the head later.

And I´m also not looking for the absolutely maximum power, just trying to build an engine thats reliable and still pulls pretty hard.

And another thing, any ideas on jetting? I was thinking about using a few steps up from stock Storm jet sizes, as a starting point. (original 38mm Mikunis)

Does anyone have the Storm jet sizes? Mains, needle jets&needles and pilots?